Protective suits are available for a range of hazardous environments, including hazardous chemicals in liquid and vapor form. One big disadvantage for workers using protective suits is that they are generally uncomfortable. To achieve good protection, the suit must be sealed to the wearer's body and therefore offers a significant barrier to heat transfer by convection, conduction, radiation and evaporation. Consequently, the temperature and humidity may rise rapidly during work. In extreme circumstances humidity approaches 100%, the body's natural cooling system stops working as no water can evaporate from the skin, blood temperature increases and, if work continues, heat stress results.
Air purifying respirators (APRs) are mounted to the suit to provide filtered air to a breathing mask for breathing and to the suit for ventilation. These devices reduce the inhalation resistance created by the filters, and increase the level of protection by creating positive pressure in the face piece. There are two types of APRs:
Constant speed respirators (PAPRs) deliver substantially constant flow rates at all times. During exhalation the air flow is wasted, and during heavy inhalation the demand often exceeds the delivery rate, resulting in negative pressure in the breathing mask and increased breathing resistance.
Demand responsive powered respirators (FPBRs) use a breathing valve and regulate fan speed to ensure positive pressure in the breathing mask under almost all conditions. During exhalation the breathing valve closes, stalling the fan, resulting in minimal wasted air.
The APR may also provide air to ventilate the suit, and in this case higher capacity battery, filters and blower are required to ensure the air flow for breathing is not compromised.